1. Field of the Invention
The invention generally relates to hybrid positioning and more specifically, describes new methods to assess the quality of a reported location from specific source of location to be used for hybrid positioning.
2. Description of the Related Art
In recent years the number of mobile computing devices has increased dramatically, creating the need for more advanced mobile and wireless services. Mobile email, walkie-talkie services, multi-player gaming and call-following are examples of how new applications are emerging for mobile devices. In addition, users are beginning to demand/seek applications that not only utilize their current location but also share that location information with others. Parents wish to keep track of their children, supervisors need to track the locations of the company's delivery vehicles, and a business traveler looks to find the nearest pharmacy to pick up a prescription. All of these examples require an individual to know his own current location or the location of someone else. To date, we all rely on asking for directions, calling someone to ask their whereabouts or having workers check-in from time to time to report their positions.
Location-based services are an emerging area of mobile applications that leverage the ability of new devices to calculate their current geographic positions and report them to a user or to a service. Examples of these services range from obtaining local weather, traffic updates and driving directions to child trackers, buddy finders and urban concierge services. These new location-sensitive devices rely on a variety of technologies that all use the same general concept. By measuring radio signals originating from known reference points, these devices can mathematically calculate the user's position relative to these reference points. Each of these approaches has its strengths and weaknesses depending upon the nature of the signals and measurements, and the positioning algorithms employed.
The Navstar Global Positioning System (“GPS”) operated by the US Government leverages about two-dozen orbiting satellites in medium-earth orbits as reference points. A user equipped with a GPS receiver can estimate his three-dimensional position (latitude, longitude, and altitude) anywhere at any time within several meters of the true location as long as the receiver can see enough of the sky to have four or more satellites “in view.” Cellular carriers have used signals originating from and received at cell towers to determine a user's or a mobile device's location. Assisted GPS (“AGPS”) is another model that combines both GPS and cellular tower techniques to estimate the locations of mobile users who may be indoors and must cope with attenuation of GPS signals on account of sky blockage. In this model, the cellular network attempts to help a GPS receiver improve its signal reception by transmitting information about the satellite positions, their clock offsets, a precise estimate of the current time, and a rough location of the user based on the location of cell towers. No distinction is made in what follows between GPS and AGPS.
All positioning systems using satellites as reference points are referred to herein as Satellite-based Positioning System (“SPS”). While GPS is the only operational SPS at this writing, other systems are under development or in planning A Russian system called GLONASS and a European system called Galileo may become operational in the next few years. All such systems are referred to herein as SPS. GPS, GLONASS and Galileo are all based on the same basic idea of trilateration, i.e., estimating a position on the basis of measurements of ranges to the satellites whose positions are known. In each case, the satellites transmit the values of certain parameters which allow the receiver to compute the satellite position at a specific instant. The ranges to satellites from a receiver are measured in terms of the transit times of the signals. These range measurements can contain a common bias due to the lack of synchronization between the satellite and receiver (user device) clocks, and are referred to as pseudoranges. The lack of synchronization between the satellite clock and the receiver (user device) clock can result in a difference between the receiver clock and the satellite clock, which is referred to as internal SPS receiver clock bias or receiver clock bias, here. In order to estimate a three dimensional position there is a need for four satellites to estimate receiver clock bias along with three dimensional measurements. Additional measurements from each satellite correspond to pseudorange rates in the form of Doppler frequency. References below to raw SPS measurements are intended generally to mean pseudoranges and Doppler frequency measurements. References to SPS data are intended generally to mean data broadcast by the satellites. References to an SPS equation are intended to mean a mathematical equation relating the measurements and data from a satellite to the position and velocity of an SPS receiver.
WLAN-based positioning is a technology which uses WLAN access points to determine the location of mobile users. Metro-wide WLAN-based positioning systems have been explored by several research labs. The most important research efforts in this area have been conducted by the PlaceLab (www.placelab.com, a project sponsored by Microsoft and Intel); the University of California, San Diego ActiveCampus project (ActiveCampus—Sustaining Educational Communities through Mobile Technology, technical report #CS2002-0714); and the MIT campus-wide location system. One example of a commercial metropolitan WLAN-based positioning system in the market at the time of this writing, is referred to herein as a WiFi Positioning System (“WPS”) and is a product of Skyhook Wireless, Inc.